This commit disables checkMulticastStats for AIX operating system.

Change-Id: If8d0fb609a0dcf75b7bb5c3871cfb6fad76a0a92
Reviewed-on: https://go-review.googlesource.com/c/144102
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>